In a process of manufacturing a steel plate such as a steel plate or a steel sheet by hot rolling, for example, in equipment shown in FIG. 8, water cooling or air cooling is applied to a steel plate (hot rolled steel plate) after hot rough rolling and hot finish rolling are performed, thus controlling the structure of the steel plate. When the steel plate is cooled to a relatively low temperature, for example, 450 to 650° C. by water cooling, the steel plate can acquire the fine ferrite or bainite structure so that the steel plate can ensure strength thereof. Accordingly, a technique which cools a steel plate by spray cooling water or laminar cooling water has been adopted in general. Recently, techniques which acquire a high cooling rate for making the structure of a steel plate finer thus enhancing strength of a steel plate have been developed vigorously.
For example, as a technique which cools a hot rolled steel plate by supplying a large quantity of columnar laminar cooling water, a technique disclosed in Japanese Patent Unexamined Publication 2002-239623 or Japanese Patent Unexamined Publication 2004-66308 is named. In this technique, cooling water is jetted to upper and lower surfaces of a steel plate at a high speed from a large number of nozzles. This technique acquires an extremely high cooling rate and is expected to manufacture a product having excellent material properties.
Also as another technique which cools a hot rolled steel plate by supplying cooling water to the steel plate, a technique disclosed in Japanese Patent Unexamined Publication 2006-35233 is named. In this technique, cooling water which is jetted from nozzles is filled in a region surrounded by a steel plate, rolls and side walls so that a pool is formed whereby a steady cooling state is acquired leading to the reduction of cooling deviation in the widthwise direction.
However, the prior art has problems in cooling ability and in ensuring cooling uniformity.
In the techniques disclosed in Japanese Patent Unexamined Publication 2002-239623 and Japanese Patent Unexamined Publication 2004-66308, cooling water which is jetted from a plurality of jetting nozzles passes through one hole or slit formed in a protective sheet arranged between a cooling water header and a hot rolled steel strip, and cooling water supplied to the steel strip is discharged through the same hole or slit. That is, the hole or the slit has both functions of a spout of nozzle and a drain outlet and, hence, as shown in FIG. 9, the flow of cooling drain is a backward flow for rod-like water flow jetted from ends of the nozzles and generates resistance to flow. Further, after reaching the steel plate, the drains rise while colliding with each other and their flow passages are bent before arriving at the drain outlet which also functions as the spout of the nozzle. Accordingly, this portion forms staying water so that the smooth flow of the drain is hindered. In this manner, it is found that the techniques disclosed in Japanese Patent Unexamined Publication 2002-239623 and Japanese Patent Unexamined Publication 2004-66308 have some difficulty in the smooth draining of cooling water supplied to a surface of a steel strip. Accordingly, to enable cooling water to surely reach the steel plate, it is necessary to apply a high injection pressure to the header to perform high-speed jetting of cooling water whereby this technique has a drawback that an equipment cost is pushed up.
Further, when a slit-shaped hole is formed, a portion of a protector plate between the slits has a narrow plate shape and, hence, the rigidity of the portion is lowered, and when a warped steel plate intrudes and collides with cooling equipment, there exists a possibility that the steel plate damages the equipment. Accordingly, although there arises no problem when a plate thickness of the steel plate which is subject to cooling processing is 2 to 3 mm, when the plate thickness becomes 15 mm or more, it is necessary to use a protector plate having a large thickness to prevent the equipment from being damaged, thus giving rise to a drawback that the formation of the slit becomes difficult.
Further, when slit-shaped holes having different sizes are formed, resistance to flow differs depending on a position of a nozzle and, hence, there also arises a drawback that the strip temperature deviation at cooling occurs in the widthwise direction of the steel plate.
The technique disclosed in Japanese Patent Unexamined Publication 2006-35233 adopts the structure where cooling water supplied to the upper surface of the steel plate forms a pool in a space surrounded by the steel plate, the roll and the side wall, and cooling water is discharged upward. Accordingly, it takes a considerable time to fill the space with cooling water and, hence, in a range of several meters from a leading edge of the steel plate, a state of cooling water becomes nonstationary, thus giving rise to a drawback that the strip temperature deviation or warping is liable to occur at the time of cooling the steel plate in the longitudinal direction.
Further, with respect to the technique disclosed in Japanese Patent Unexamined Publication 2006-35233, a case where the side wall is not provided is also disclosed. In this case, as indicated by an arrow indicated by a dotted line in FIG. 12, drain flows on a guide plate (indicated as a dividing wall in place of the guide plate in FIG. 12) in the direction toward a widthwise edge portion of the steel plate. In the technique disclosed in Japanese Patent Unexamined Publication 2006-35233, an end of the cooling nozzle is arranged above the guide plate and, hence, the widthwise directional flow of the drain interferes with cooling water jetted from the cooling nozzle.
The closer to the edge portion of the steel plate in the widthwise direction, the larger the widthwise flow of the drain becomes and, hence, the closer to the edge portion of the steel plate in the widthwise direction, the larger the interference becomes. Accordingly, a part of or the whole cooling water jetted from the cooling nozzle cannot reach the upper surface of the steel plate so that the uniform cooling in the widthwise direction cannot be achieved.
Further, in all techniques disclosed in Japanese Patent Unexamined Publication 2002-239623, Japanese Patent Unexamined Publication 2004-66308 and Japanese Patent Unexamined Publication 2006-35233, cooling water is jetted from above and below the steel plate. In a case where a steel plate to be cooled is not present such as a case where the steel plate has not yet entered the inside of a cooling device or a case where there are regions outside a plate width of a steel plate to be cooled, cooling waters which are jetted from above and below the steel plate collide with each other and splash to a periphery around the steel plate. Splashed water breaks a flux of cooling water jetted from the surrounding cooling nozzles thus giving rise to a drawback that stable cooling ability cannot be assured at a leading edge, a tailing edge and both edges of the steel plate in the widthwise direction.
Further, there may be a case where splashed water stays on the steel plate before the leading edge of the steel plate reaches a zone where cooling water is supplied and cools the leading edge of the steel plate, and there may be also a case where splashed water stays on the steel plate even after the tailing edge of the steel plate passes the zone where cooling water is supplied and cools the tailing edge of the steel plate. In such a case, the uniform cooling in the longitudinal direction cannot be achieved. Further, due to splashing of cooling water to the periphery around the steel plate, there exists a possibility that the measurement using various sensors cannot be performed or the maintenance property of peripheral equipment is deteriorated.
It could therefore be helpful to provide a technique which uniformly cools a hot rolled steel plate at a high cooling rate or at high thermal transmissivity when cooling water is supplied to an upper surface of the hot rolled steel plate or to a lower surface of the hot rolled steel plate.